Gene expression levels are influenced by the interactions of transcription factors with proteins that are present in general transcription complexes. Transcription factors generally have an activation domain and a DNA binding domain. In addition, other non-DNA binding proteins known as coactivators interact with transcription factors and transcription complex proteins to further stimulate transcription. The proteins PvAlf of Phaseolus vulgaris (Bobb et al. (1995) Plant Journal 8:101-113), ABI3 of Arabidopsis thaliana (Giraudat et al. (1992) Plant Cell 4:1251-1261), and Vp1 of maize (McCarty et al. (1991) Cell 66:895-905) and rice (Hattori et al. (1994) Plant Molecular Biology 24:805-810) are related proteins that are involved in the regulation of transcription. They have been called transcription factors, though it is unclear whether they actually can bind to DNA. They may bind to DNA in conjunction with another protein, or may actually be a coactivator type of regulator. In any case, these related proteins are stimulators of transcription. Due to the uncertainty in classification, we refer to these proteins as transcription activators, which could either be transcription factors or transcription coactivators.
PvAlf is expressed specifically during seed development. The highest levels of expression of PvAlf mRNA are at the time when seed storage protein expression begins. In transient assays in bean cotyledons, PvAlf positively regulated the expression of the phaseolin and phytohemagglutinin promoters, the promoters of two abundantly expressed seed storage protein genes (Bobb et al., (1995) Plant Journal 8:101-113). The activation response was shown to be mediated through sequences that are conserved in these and other seed storage protein gene promoters, called the Ry-repeats, as well as an associated CCAC sequence (Bobb et al. (1997) Nucleic Acids Res. 25:641-647). The N-terminal 243 amino acids of PvAlf were shown in transient assays in bean cotyledons to function as an activation domain, when fused to the DNA binding domain of the yeast Gal4 transcription factor (Bobb et al. (1995) Plant Journal 8:101-113).
The PvAlf-related ABI3 gene product of Arabidopsis is also involved in the regulation of seed storage protein genes. Mutants in the ABI3 gene cause a reduction or loss of expression of the 2S and 12S seed storage proteins (Nambara et al. (1994) Plant Cell Physiol. 35:509-513; Parcy et al. (1994) Plant Cell 6:1567-1582). Mutants in the related Vp1 gene of maize also cause reduction in seed storage protein expression (Kriz et al (1990) Plant Physiol. 92:538-542). In transient assays in maize protoplasts, Vp1 activated the maize Em promoter, the promoter from a gene expressed during embryo development (McCarty et al. (1991) Cell 66:895-905). The activation domain of Vp1 was localized to the N-terminal 121 amino acids in Gal4 fusion experiments using transient assays (McCarty et al. (1991) Cell 66:895-905). Like PvAlf, ABI3 and Vp1 expression is specific to seed development.
To date, no PvAlf, ABI3 or VP1 homologs have been reported in Glycine max.